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I have two mint plants that have grown without much fanfare. Occasionally I do notice the odd leaf with very dark brown edges, almost black. Some leaves also appear to have a powdery white substance on their leaves, leaving them pale in appearance. It does not rub off on my fingers, though.

I've watered my plants consistently every day except on wet or overcast days. The frequency of blackened leaves has increased recently, although, yesterday, I forgot to water the plant and I noticed all it's leaves had wilted heavily.

These plants get a full day's sun, save for morning time. The weather is generally sunny and mild year round (São Paulo, Brazil).

I may have been too generous with fertiliser than needed when I repotted them about six weeks ago.

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Mint plant with brown leaves Brown mint leaves

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Like your coriander, you may be overwatering your mint as well. Although mint likes moist soil, the key here is "moist" and not "wet." Unless you are experiencing very high temps and very low humidity there, then every day is probably too often to be watering your plants.

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  • How do we explain the fact that the plant wilted having not been watered for two days, though? Could the roots be rotten?
    – Mohamad
    Commented Dec 17, 2013 at 16:31
  • Yes, you could have some rotten roots. It might not be a bad idea to either dig around in the bottom of the pot or unpot the plant briefly to see what is going on. You may also be able to tell by smelling the soil.
    – TeresaMcgH
    Commented Dec 17, 2013 at 16:48
  • By the way, if the roots are rotten, you may be able to salvage the mint by taking cuttings and starting over with new soil.
    – TeresaMcgH
    Commented Dec 17, 2013 at 16:49
  • I'm going to tentatively accept this. I dug around the soil yesterday and drained any excess water. The plants seems to be doing better today. Let's see how they progress over the next couple of weeks.
    – Mohamad
    Commented Dec 17, 2013 at 19:19
  • Speaking as someone who fights constant mint rust, that doesn't look like mint rust. I think Teresa is right that it's overwatering.
    – FuzzyChef
    Commented Oct 3, 2014 at 17:15

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